This article only applies for mailboxes setup as Exchange mailboxes on your iPhone, iPod touch or iPad. If you are unsure how to setup your Exchange mailbox on your iPhone, iPod touch or iPad, please see the article “How do I set up an Exchange mailbox on an iPhone, iPod touch or iPad“.
When setting an Exchange mailbox up on an iPhone, iPod touch or iPad, the default setting to sync the last 3 days of Mail to your device. You can change this as follows:
Go to Settings > Mail, Contacts, Calendars and select your Exchange mailbox.
On the next screen, select “Mail Days to Sync”.
On the next screen, simply select number of days of mail you want to keep synced your iPhone, iPod touch or iPad.
If you want to sync ALL your mail, just select “No Limit”.